This invention relates to a method of producing a composite cutting insert and a twist drill incorporating the insert, and to a cutting insert and a twist drill formed by the method.
Twist drills cutting inserts formed from polycrystalline diamond (PCD) composite compacts are known in various forms. One known twist drill of this type incorporates a cutting insert which is generally wedge- or chevron-shaped, which comprises a central layer of polycrystalline diamond with a thin tungsten carbide layer on each side thereof. The cutting insert is fitted into a diametral slot in the end of the twist drill body so that the tip of the wedge or chevron is aligned with the central axis of the drill. The resulting twist drill resembles a conventional masonry drill, but has respective chisel edges defined by the diamond material of the insert.
The above described cutting inserts have a number of drawbacks. Firstly, the sandwich-type double-skinned PCD material, with dual tungsten carbide layers, is relatively expensive. The sandwich material also suffers from a relatively high level of internal stresses, which may require the use of carbide as a matching material in the drill body. This considerably increases the cost of the finished twist drill. Also, inserts of this kind are prone to breakage, particularly with drills of longer than 100 to 125 mm.